This invention relates to a communication system for transmitting and receiving information over a telephone line or the like. More specifically, the invention relates to a device which expands the capacity of the line by permitting the simultaneous transmission of two or more voice conversations over the same telephone line or similar voice grade circuits.
The invention is an improvement over voice signal processing such as those disclosed in PCT Patent Application No. PCT/US84/00848, published Dec. 20, 1984 under International Publication No. WO84/04989 and PCT Patent Application No. PCT/US84/01315, published on Feb. 28, 1985 under International Publication No. WO85/00944.
The first patent application identified above discloses an apparatus for selecting and discarding or duplicating alternate pitch periods of an analog voice signal.
A peak detector is used to identify glottal pulses in the analog voice signal. The periodicity of the glottal pulses is used to determine jump intervals in a microprocessor controlled system which stores successive samples of the analog voice signal. By using this method and apparatus alternate pitch periods can be selected to create a compressed version of the speech signal. Similarly, the speech signal can be expanded by using jump logic to duplicate successive speech intervals or glottal epoches received.
The second patent referenced above, discloses a method of stacking compressed analog speech signals and related communication signals so that two speech signals can be transmitted on the same telephone line.
As shown in the figures to that patent, particularly FIGS. 3 and 4, stacking is achieved with intermediate frequency techniques by modulating one of the two analog voice signals with a 455 kilohertz carrier and then demodulating it with a 451.8 kilohertz carrier prior to transmission. A similar process is utilized at the receiving end of the telephone line after which the signal is expanded for normal audio processing.